The present invention relates to a process and an apparatus for the treatment of silicon tetrachloride or germanium tetrachloride contaminated with at least one hydrogen compound which is difficult to separate off by distillation.
Silicon tetrachloride (SiCl4) is used, inter alia, for the production of optical waveguides. SiCl4 of very high purity is required for these applications. In particular, hydrogen-containing impurities are a critical disadvantage, even when they are present in only ppm quantities.
Among hydrogen-containing impurities in silicon tetrachloride, a distinction has to be made between impurities which are difficult and easy to separate off. HCl, for example, can be separated off from the silicon tetrachloride down to a concentration in the range <1 ppm by weight by simple fractional distillation. On the other hand, hydrocarbons in particular, but also chlorinated hydrocarbons and corresponding compounds such as silanes containing alkyl groups, cannot be separated off down to concentrations in the range <1 ppm by weight by simple fractional distillation.
Efforts are likewise made to make germanium tetrachloride available in the purest form possible, in particular in high-purity form.
Possible ways of removing hydrocarbons, chlorinated hydrocarbons and corresponding compounds such as silanes containing alkyl groups from silicon tetrachloride have been known for a long time.
Thus, silicon tetrachloride containing the abovementioned components can be treated as described in U.S. Pat. No. 4,372,834 and EP 0 488 765 A1 with UV radiation in the wavelength range from 200 to 380 nm in the presence of chlorine and the chlorination products obtained can subsequently be separated off from SiCl4 by fine distillation. A significant disadvantage of this process is that the plant components come into contact with chlorine gas which, according to EP 0 488 765 A1, is added in considerable amounts and are thus subjected to particularly severe corrosion, which inevitably leads to frequent downtimes of the plant. Furthermore, the chlorine to be added likewise has to meet very high purity requirements. Both result in high operating costs for the plant. Another particular disadvantage which may be mentioned is the particularly poor energetic efficiency of UV radiation sources proposed, for example, by EP 0 488 765 A1. This results in particularly long treatment times, which likewise leads to high costs.
A general process for purifying halogen and hydrogen compounds of silicon is likewise known (DE-B 10 58 482). In this, chlorosilanes and bromosilanes can be treated by addition of a reducing agent, e.g. hydrogen, silicon, sodium, aluminum or zinc, and action of a gas discharge, in particular a dark gas discharge, resulting in formation of relatively high molecular weight compounds in which the elements carbon, boron or phosphorus can be incorporated in relatively high molecular weight compounds of chlorosilicon by free radical formation and combination of free radicals present, and these relatively high molecular weight compounds are separated off by distillation. A particular disadvantage of this process is the need to add a reducing agent. In particular, DE-B 10 58 482 teaches addition of hydrogen as reducing agent in the purification of an SiCl4 fraction.